Wool exhibits by nature a pronounced yellow color and also on exposure to light , alkali or by microbial degradation. Commercially, wool bleaching is carried out using either an oxidative or a reductive system, or a combined oxidation/reduction process. Oxidative bleaching in the dyebath is also possible.

In general, oxidative bleaching with hydrogen peroxide gives superior whiteness over reductive methods. Recent research also revealed the use enzymes to enhance the whiteness of bleached wool.

Wool cannot be bleached with sodium hypochlorite solutions, as for cotton, since it is extensively damaged to the point at which it even dissolves in the solution.

Oxidative Bleaching Method
A batch treatment with hydrogen peroxide is used for most bleaching applications.An activator (eg an alkali) is normally added to increase the rate of bleaching. Typically, wool is bleached at pH 8–9 for 1 h at 60◦C with a stabilized solution of hydrogen peroxide (0.75% w/w). It is generally accepted that, under alkaline conditions, the active bleaching species is the perhydroxy anion (OOH− ), the formation of which is encouraged by higher pH .

Peroxide bleaching of wool under mild acidic conditions (pH 5–6) can also be carried out using a peracid activator such as Prestogen W (BASF) or citric acid . As wool sustains some damage in the presence of alkali, this method is useful for bleaching delicate fabrics.

An undesirable side effect is the rapid decomposition of hydrogen peroxide to water and oxygen, a reaction catalyzed by transition-metal ions. A stabilizer, which sequesters these ions, is used to prevent this side reaction occurring. The most common stabilizers for alkaline wool bleaching are phosphates, particularly tetrasodium pyrophosphate. However, recent concerns over phosphates in effluents from textile treatment have led to the development of alternative stabilizers based on silicates.

Heavily pigmented fibers, such as Karakul wools, require a more severe approach known as mordant bleaching. In this method, the wool is treated with a metal salt and then with hydrogen peroxide. In the first step, the melanin pigment in the wool preferentially absorbs the metal cations; and in the second step, the cations catalytically decompose the peroxide to produce highly aggressive hydroxyl free radicals, which selectively attack and bleach the melanin.

Reductive Bleaching
The two most popular chemicals used for reductive bleaching of wool are stabilized sodium dithionite and thiourea dioxide. Most reductive bleaching of wool is carried out using stabilized dithionite (2–5g/L) at pH 5.5–6 and 45–65◦C for 1 h. Thiourea dioxide is more expensive than sodium dithionite, but is an effective bleach when applied (1–3 g/L) at 80◦C and pH 7 for 1 h. Whiter fabrics are produced when oxidative bleaching is followed by a reductive process—this is often referred to as “full bleaching.”

Reductive Bleaching with Sulfur Dioxide
In the early days sulfur dioxide was used to bleach wool , but disadvantage of this method of bleaching wool was that the white was not permanent when exposed to sun and air.

Full Bleaching Of Wool
However, with even the best oxidative bleaching processes, there is a limit to the whiteness that may be achieved on wool, within the limits of acceptable fibre damage.Whiter fabrics are produced when oxidative bleaching is followed by a reductive process(combined oxidative/reductive bleaching treatments)—this is often referred to as “full bleaching.”

Shrink Resistance Treatment of Wool
Felting in garments and fabrics that leads to excessive shrinkage is, however, undesirable. It occurs when the wet material is subjected to severe mechanical action, for example, in laundering or tumble drying . Shrink-resist treatments are directed at preventing felting shrinkage, whereas minimization of relaxation shrinkage requires careful control during fabric finishing.

The term shrink-resistant is preferred to “shrinkproofed,Shrink resist treatment of wool adds consumer desired properties to this superior natural fibre, the improved processing technology allows processors and retailers to offer fabrics and garments with added value and performance. The improved performance offered by the Total Easy Care range of processes includes machine washability and tumble dry qualities.

Principle of SR TreatmentA variety of methods produce wool fabrics that withstand repeated washing without shrinkage and felting. They are particularly important for knitted woollens and worsted fabrics.

Two main principles are used:

Modification of the scale structure of the fibre cuticle to decrease the directional friction effect – this can be achieved by chemical treatments that either partially remove the scales or cover them with a smooth film of polymer;

Reduction of fibre mobility by adhesion of fibres and yarns at their points of contact and by decreasing fibre elasticity by means of intermolecular crosslinking.

Chlorine SR Treatment
Chlorine-Based Shrink-Resist Treatments. The principal oxidizing agent used in degradative shrink-resist treatments is chlorine. Free chlorine reacts very rapidly with wool; hence, it is difficult to treat a mass of wool fibers evenly. Two different types of chlorination methods are used commercially: continuous treatment and batch treatment. In the continuous method, top or loose wool is reacted with an aqueous solution of chlorine gas for a short time (<30 s). The batch treatment method involves a longer treatment time (5–30 min) with a less reactive chlorinating agent, such as DCCA (N,N -dichloroisocyanuric acid). Batch treatments aremainly used on garments and fabrics but can also be applied to loose wool or tops. Generally, in both continuous and batch treatments a reactive polymer, usually cationic, is applied after the chlorination step.

Chlorine Free SR treatment
Chlorine-Free Shrink-Resist Treatments. Commercially, the only other oxidizing agent used to any extent is permonosulfuric acid (PMS; HOOSO3H). This is used in the form of its potassium triple salt, containing potassium sulfate and bisulfate. It is employed in batch processes at elevated temperatures, because it reacts more slowly with wool than chlorine. The process sequence is similar to that used for chlorine-based treatments. It involves degradative oxidation with PMS, followed by neutralization with sodium sulfite and then application of a resin. Unlike chlorine, however, PMS does not remove the bound lipid or oxidize cystine to cysteic acid. The main product of the reaction is cystine sulfonic acid or Bunte salt groups.

Plasma Treatment in SR wool
There is an enormous potential in the plasma treatment of natural fibre fabrics. Plasma treatment has proved to be successful in the shrink-resist treatment of wool with a simultaneously positive effect on the dyeing and printing.

Additive Shrink-Resist Treatments

The principal additive shrink-resist treatment for wool fabrics uses the polymer Synthappret BAP (Bayer AG). This is a poly(propylene oxide) polyurethane, containing reactive carbamoyl sulfonate (bisulfite adducts of isocyanate) groups, ie NHCOSO3 − Na+. An aqueous solution of this polymer is padded onto woven fabrics. The polymer cross-links on drying to form flexible linkages between fibers and yarns. Other polymers may be applied at the same time to modify the handle.

Sharing Knowledge: Students, teachers and professionals can publish your article here. It is a platform to express your knowledge throughout the world. For details: Submit Article

Editor-in-Chief:

Mazharul Islam Kiron is a textile consultant and researcher on online business promotion. He is working with one European textile machinery company as a country agent. He is also a contributor of Wikipedia.